DB SPL

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which corresponds to a root-mean-square (rms) pressure amplitude
of Pa, or
about Pa, as
listed above. The wave
impedanceof air plays the role of “resistor” in relating
the pressure- and intensity-based references exactly analogous to
the dBm case
discussed above.

Since sound is created by a time-varying pressure, we
compute sound levels in dB-SPL
by using the average intensity (averaged over at least one
period of the lowest frequency
contained in the sound).

Table 4.1
gives a list list of common sound levels and their dBequivalents4.5 [9]:

In my experience, the “threshold of pain” is most often defined as
120 dB.

The relationship between sound amplitude and actual
loudness is complex [10].
Loudness is a perceptual dimension while sound amplitude is
physical. Since loudness sensitivity is closer to logarithmic than
linear in amplitude (especially at moderate to high loudnesses), we
typically use decibels to
represent sound amplitude, especially in spectral displays.

The sone amplitude scale is
defined in terms of actual loudness perception experiments
[10].
At 1kHz and above, loudness perception is approximately logarithmic
above 50 dB SPL or so. Below that, it tends toward being more
linear.

The phon amplitude scale is
simply the dB scale at
1kHz [10,
p. 111]. At other frequencies, the amplitude in phons is
defined by following the equal-loudness curve over to 1 kHz and
reading off the level there in dB SPL. In other words, all pure
tones have the same loudness at the same phon level, and 1 kHz is
used to set the reference in dB SPL. Just remember that one phon is
one dB-SPL at 1 kHz. Looking at the Fletcher-Munson equal-loudness
curves [10,
p. 124], loudness in phons can be read off along the vertical
line at 1 kHz.

Classically, the intensity level of a sound wave is its dB SPL level, measuring the peak
time-domain pressure-wave
amplitude relative to watts per centimeter squared (i.e., there is no
consideration of the frequency domain here at all).

Another classical term still encountered is the sensation
level of pure tones: The sensation
level is the number of dB SPL above thehearing threshold at
that frequency [10,
p. 110].